As previously noted, it obviously implies “judge not lest ye be judged” and “first remove the plank from your own eye” (clean yer own house, ya bum!), but it may have another meaning.
If Jesus did say “without sin”, rather than “perfect”, then perhaps he was using sin’s orignial root meaning of “miss the target” intentionally in reference to those assembled, in order to illustrate another point: none of the assembled would have been able to hit the sin in the woman, rather than hitting the woman (hate the sin but love the sinner). Thus, not only would the stoning be wrong, but it would actually inherently be impossible for it to achieve the goal and, as that goal was to remove the missing of the mark from the woman, failing to do so was just compounding the failure, and demanding that the stoning take place was an even greater folly. The utter absurdity of it would then have implied, to anyone who had heard Jesus’ teachings, that the act, and the law which called for it, were pointless, and that the Pharisees, along with any who obeyed them, were fools.
I suspect that any (learned) man there would have recognized the “miss the mark” reference, and likely strongly reacted if they recognized the context being directly relevant; that sort of thing carries the weight of truth. Whether only Jesus’ disciples would have caught the “hate the sin” reference, or if others would have heard it too, is another question.
This all assumes that he said “without sin”, of course. If he didn’t, then I’m missing the mark! 
Tho the Greek has it as “he who is perfect”, that was not written until hundreds of years later, and Jesus would probably not have been speaking Greek in discussing a matter of Jewish law that the Pharisees brought up, as it seems like it would have placed him in the position of outsider, and thus undermined his position.
Does anyone know where “without sin” was first put as the translation, and why that wording was chosen?